Nickel positive electrode for alkaline storage battery and sealed nickel-hydrogen storage battery using nickel positive electrode

ABSTRACT

A nickel positive electrode for an alkaline storage battery comprises an active material mixture mainly composed of nickel hydroxide and a conductive support. The active material mixture contains at least one member selected from the group consisting of cobalt, cobalt hydroxide and cobalt oxide, and carbon powder having a lattice constant &#34;d&#34; of a (002) plane such that 3.35 Å&lt;d≦3.45 Å.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved nickel positive electrodefor an alkaline storage battery, and also to a sealed typenickel-hydrogen storage battery using the nickel positive electrode.

2. Description of the Related Art

Recent trend for high value-added and down-sized portable apparatus hassignificantly enhanced the requirement for high energy density secondarycells. Development of new secondary cells having a high energy densityis also highly required as a battery of electric vehicles. Alkalinestorage batteries recently developed to meet these demands include ahigh-capacity nickel-cadmium storage battery with a conventionalsintered nickel positive electrode, and a high energy-densitynickel-cadmium storage battery with a foamed nickel positive electrodehaving a capacity 30 to 60% higher than that of the conventionalelectrode. Another example is a nickel-hydrogen storage battery using ahydrogen storage alloy as a negative electrode, which has a highercapacity than the nickel-cadmium storage batteries.

In such high-capacity alkaline storage batteries, nickel hydroxidepowder is closely packed into a sintered nickel porous substrate, athree-dimensional foamed nickel porous substrate having the highporosity of not less than 90% or a nickel fiber porous substrate inorder to improve the energy density of the positive electrode. Theclosely packed nickel hydroxide has improved the energy density to 450to 500 mAh/cm³ for the recent sintered nickel positive electrodes and to550 to 650 mAh/cm³ for the foamed nickel positive electrodes, comparedto the energy density of 400 to 450 mAh/cm³ for the conventionalsintered nickel positive electrodes.

In such positive electrodes prepared by closely packing nickel hydroxidepowder in a sintered nickel porous substrate, a foamed nickel poroussubstrate or a nickel fiber porous substrate, the packing density isincreased by application of pressure. Application of the pressure,however, causes expansion of electrode plates and compresses separatorseach placed between a positive electrode and a negative electrode in thecourse of repetitive charging and discharging. The compression of theseparators presses out an electrolyte solution included in theseparators which makes a significant contribution to charge anddischarge characteristics, thus deteriorating the dischargecharacteristics. A battery is sometimes discharged to approximately 0 Vby an accidental continuous power-on of a portable apparatus. Repeatedcharging and discharging of such a battery significantly lowers thedischarge voltage and increases the internal impedance.

The positive electrode prepared by closely packing nickel hydroxidepowder in a sintered nickel porous substrate, a foamed nickel poroussubstrate or a nickel fiber porous substrate has a high energy densityat ordinary temperatures but has a relatively low energy density in ahigh-temperature atmosphere. Namely, the merit of the high energydensity can not be exerted in a wide temperature range. Charging under ahigh-temperature atmosphere generates oxygen simultaneously with adischarge reaction of converting nickel hydroxide to nickeloxyhydroxide. In other words, this decreases the overvoltage forevolution of oxygen at the positive electrode and prevents the dischargereaction of converting nickel hydroxide to nickel oxyhydroxide, thuslowering the utilization of nickel hydroxide.

Several methods have been proposed to solve the above-mentionedproblems:

(1) A method of adding cadmium oxide powder or cadmium hydroxide powderto a positive electrode;

(2) A method of making a cadmium oxide contained in nickel hydroxidepowder (disclosed in Japanese Laid-Open Patent No. 61-104565); and

(3) A method of adding a powdery compound of a IIa group element, suchas, calcium hydroxide to a sintered nickel positive electrode (disclosedin Japanese Laid-Open Patent No. 48-46841 and U.S. Pat. No. 3,826,684).

An active material mixture supported by a conductive support such as asintered nickel porous substrate, a foamed nickel porous substrate or anickel fiber porous substrate includes metal cobalt powder for enhancingthe utilization of the active material and metal nickel powder as aconductive agent. Cobalt hydroxide or cobalt oxide may be used in placeof the metal cobalt powder. In the positive electrode including such anactive material mixture, repeated charging and discharging cycles havinga large discharge depth, for example, discharge to approximately 0 V,lower the discharge voltage and increase the internal impedance toshorten the cycle life. This problem can not be solved by simplyadjusting the quantity of metal cobalt powder or metal nickel powderadded to the positive electrode.

In the method (1) or (2), a cadmium oxide is included in or mixed withnickel hydroxide powder to improve the utilization of nickel hydroxidein a high-temperature atmosphere. Addition of the cadmium oxide,however, improves the utilization of nickel hydroxide only toapproximately 80% in the high-temperature atmosphere. A larger quantityof the cadmium oxide included in or mixed with nickel hydroxide powderis essential for further improvement of the utilization of nickelhydroxide in the high-temperature atmosphere. While actually improvingthe utilization of nickel hydroxide to approximately 90% in thehigh-temperature atmosphere, the large quantity of the cadmium oxidelowers the utilization of nickel hydroxide at ordinary temperatures.Addition of heavy metal, cadmium compounds is not favorable forprotection of the environment.

In the method (3), a sintered nickel positive electrode is immersedfirst in an aqueous solution of calcium nitrate and then in an aqueoussolution of sodium hydroxide. Calcium hydroxide thus precipitated isadded to the positive electrode to improve the utilization of nickelhydroxide in a high-temperature atmosphere. Like the above methods (1)and (2), addition of calcium hydroxide improves the utilization ofnickel hydroxide in the high-temperature atmosphere while lowering theutilization of nickel hydroxide at ordinary temperatures. In the method(3), calcium hydroxide is added to the sintered nickel positiveelectrode by immersing the electrode in a calcium nitrate solution. Thiscauses the residual nitrates to exist in the nickel positive electrode.In a sealed battery with such a sintered nickel positive electrode, theresidual nitrates undesirably increase the self discharge.

Addition of calcium hydroxide powder to the paste-type nickel positiveelectrode does not improve the utilization at ordinary and hightemperatures.

SUMMARY OF THE INVENTION

One object of the present invention is accordingly to provide animproved nickel positive electrode for alkaline storage batteries, whichmanufactured by a simple process, but has excellent dischargecharacteristics and realizes favorable utilization of nickel hydroxidein an atmosphere of a wide temperature range.

Another object of the present invention is to provide a nickel positiveelectrode for alkaline storage batteries, which has a sufficiently longcycle life even under conditions of charging and discharging cycles witha large discharge depth.

Still another object of the present invention is to provide a sealednickel-hydrogen storage battery having a high energy density and asufficiently long cycle life.

According to the present invention, a nickel positive electrode for analkaline storage battery comprises an active material mixture mainlycomposed of nickel hydroxide and a conductive support, and the activematerial mixture contains at least one member selected from the groupconsisting of cobalt, cobalt hydroxide and cobalt oxide, and carbonpowder having a lattice constant "d" of a (002) plane such that 3.35Å<d≦3.45 Å.

In a preferable embodiment of the present invention, the active materialmixture further contains a powdery compound of at least one elementselected from the group consisting of Ca, Sr, Ba, Cu, Ag and Y.

The present invention is also directed to a nickel positive electrodefor an alkaline storage battery which includes an active materialmixture mainly composed of nickel hydroxide and a conductive support,and the active material mixture contains at least one member selectedfrom the group consisting of cobalt, cobalt hydroxide and cobalt oxide,nickel powder having a specific surface area of from 0.1 to 3 m² /g andan average particle diameter of from 0.1 to 15 micrometer, and a powderycompound of at least one element selected from the group consisting ofCa, St, Ba, Cu, Ag and Y.

Preferable examples of the compound include Ca(OH)₂, CaO, CaF₂, CaS,CaSO₄, CaSi₂ O₅, CaC₂ O₄, CaWO₄, SrCO₃, Sr(OH)₂, BaO, Cu₂ O, Ag₂ O, Y₂(CO₃)₃ and Y₂ O₃.

A sealed nickel-hydrogen storage battery of the present inventionincludes any nickel positive electrode described above, a negativeelectrode including a hydrogen storage alloy which electrochemicallyabsorbs and desorbs hydrogen, a separator, an electrolyte solutionconsisting of an alkaline aqueous solution and a sealed container havingpositive and negative terminals and a resettable safety valve.

In a preferable embodiment of the present invention, the content of thecarbon powder in the active material mixture is from 0.1 to 8 parts byweight per 100 parts by weight of the nickel hydroxide.

The preferable carbon powder is flake graphite powder.

In another preferable embodiment of the Invention, the content of thepowdery compound in the active material mixture selected from the groupconsisting of Ca(OH)₂, CaO, CaF₂, CaS, CaSO₄, CaSi₂ O₅, CaC₂ O₄, CaWO₄,SrCO₃, Sr(OH)₂, BaO, Cu₂ O, Ag₂ O, Y₂ (CO₃)₃ and Y₂ O₃, is from 0.1 to 5parts by weight per 100 parts by weight of the nickel hydroxide.

The content of the nickel powder is preferably from 0.1 to 8 parts byweight per 100 parts by weight of the nickel hydroxide.

While the novel features of the present invention are set fourthparticularly In the appended claims, the invention, both as toorganization and content, will be better understood and appreciated,along with other objects and features thereof, from the followingdetailed description taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the discharge voltage plotted against the lattice constantof carbon powder included in each positive electrode;

FIG. 2 is a vertical sectional view schematically illustrating a sealednickel-hydrogen storage battery embodying the invention;

FIG. 3 shows the cycle life characteristics of nickel-hydrogen storagebatteries including a variety of positive electrodes;

FIG. 4 shows the relationship between the utilization and the latticeconstant of carbon powder included in each positive electrode at variouscharge temperatures;

FIG. 5 shows the relationship between the charge temperature and theutilization in a variety of positive electrodes;

FIG. 6 shows the relationship between the utilization and the specificsurface area of nickel powder included in each positive electrode atvarious charge temperatures;

FIG. 7 shows the relationship between the utilization and the averageparticle diameter of nickel powder included in each positive electrodeat various charge temperatures; and

FIG. 8 shows the relationship between the charge temperature and theutilization in a variety of positive electrodes.

It will be recognized that some or all of the Figures are schematicrepresentations for purposes of illustration and do not necessarilydepict the actual relative sizes or locations of the element shown.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a nickel positive electrode, ormore specifically to a paste-type nickel positive electrode comprisingan active material mixture, which contains nickel hydroxide as a primarycomponent, at least one member selected from the group consisting ofcobalt, cobalt hydroxide and cobalt oxide, and carbon powder having alattice constant of a (002) plane as specified above. The positiveelectrode of the present invention contains the cobalt or cobaltcompound for constituting a conductive network and the carbon powder forsupporting the conductive network, which efficiently improve theconductivity and the charge efficiency. The carbon powder specified asabove inhibits oxidation of carbon (C+O₂ →CO₂) at the time of charging,thus allowing the positive electrode to maintain the high conductivity.The enhanced conductive network allows the positive electrode to have afavorable long cycle life even under conditions of expansion andcontraction of an electrode plate due to discharge with a largedischarge depth up to approximately 0.1 V and subsequent repetition ofcharging and discharging cycles.

The paste-type nickel positive electrode of the present inventionfurther contains at least one member selected from the group consistingof Ca(OH)₂, CaO, CaF₂, CaS, CaSO₄, CaSi₂ O₅, CaC₂ O₄, CaWO₄, SrCO₃,Sr(OH)₂, BaO, Cu₂ O, Ag₂ O, Y₂ (CO₃)₃ and Y₂ O₃. These compounds areadsorbed to the surface of the nickel hydroxide functioning as an activematerial, and increase an overvoltage for evolution of oxygen, which isa competitive reaction in charging under a high-temperature atmosphereexpressed as reaction formula (1). As a result, a charge reaction ofnickel hydroxide to nickel oxyhydroxide expressed as reaction formula(2) sufficiently proceeds to improve the utilization of the nickelpositive electrode in the high-temperature atmosphere:

    2OH.sup.- →1/20.sub.2 +H.sub.2 O+e.sup.-            (1)

    Ni(OH).sub.2 +OH.sup.- →NiOOH+H.sub.2 O+.sup.-      (2)

(2)

The paste-type nickel positive electrode of the present invention whichcontains at least one of cobalt, cobalt hydroxide and cobalt oxide mayfurther contain carbon powder having a lattice constant of a (002) planeas specified above, or nickel powder having a specific surface area andan average particle diameter as specified above and at least one powderycompound such as Ca(OH)₂. The synergism of the cobalt or cobalt compoundfor constituting a conductive network and the carbon powder or mixtureof the nickel powder and the powdery compound such as Ca(OH)₂ forsupporting the conductive network increases the overvoltage forevolution of oxygen, which is a competitive reaction in charging under ahigh-temperature atmosphere. This results in improvement of theutilization of the nickel positive electrode under the high-temperatureatmosphere in the same manner as above. The improved conductivitymaintains the high utilization of the nickel positive electrode under anatmosphere of ordinary or low temperatures.

The structure described gives the nickel positive electrode preferabledischarge characteristics and a sufficiently long cycle life, andimproves the utilization of nickel hydroxide under an atmosphere of awide temperature range from the ordinary to high temperatures. Anappropriate amount of the powdery compound does not adversely affect theutilization of nickel hydroxide. The present invention accordingly givesa positive electrode having preferable discharge characteristics andexcellent utilization of the active material in a wide temperaturerange, and an improved sealed nickel-hydrogen storage battery using thepositive electrode.

The preferable range of the content of the carbon powder included in theactive material mixture is from 0.1 to 8 parts by weight per 100 partsby weight of the nickel hydroxide. The content smaller than the aboverange does not properly exert the effects whereas the content greaterthan the above range lowers the energy density. Flake graphite powderwhich gives significantly high conductivity is preferable as the carbonpowder.

The conductive support included in the positive electrode may be afoamed nickel porous substrate, a nickel fiber porous substrate, asintered nickel porous substrate or a three-dimensional porous substratesuch as punching metal. Other examples of the conductive support includea nickel flat plate or a nickel-plated iron foil. In the case of thefoamed nickel porous substrate, the preferable surface density of theporous substrate is determined to be from 200 to 700 g/m² from theviewpoint of current collecting characteristics.

From the standpoints of the uniform filling and formation of theconductive network, the nickel hydroxide is preferably spherical nickelhydroxide having an average particle diameter of from 1 to 30micrometer.

The preferable content of the powdery compound, such as Ca(OH)₂,included in the active material mixture is from 0.1 to 5 parts by weightper 100 parts by weight of the nickel hydroxide. The content smallerthan the above range does not properly exert the effects whereas thecontent greater than the above range lowers the energy density. Thecontent of the nickel powder is preferably from 0.1 to 8 parts by weightper 100 parts by weight of the nickel hydroxide.

The structure and effects of the invention will be more apparent throughthe following description of the preferred examples.

EXAMPLE 1

Spherical nickel hydroxide powder, cobalt powder, cobalt hydroxidepowder and carbon powder having a variety of (002) plane-latticeconstants "d" were weighed at a weight ratio of 100:3:2.5:4,sufficiently mixed with each other and kneaded with water to a paste.Subsequently, the paste was applied to a foamed nickel substrate used asa support having a thickness of 1.6 mm, a porosity of 95% and a surfacedensity of 600 g/m². Each substrate filled with the paste was dried,pressure-molded and immersed in an aqueous dispersion of a fluorocarbonresin powder. The immersed substrate was dried again, and cut into anickel positive electrode having a size of 90×70 mm and a thickness of0.9 mm, with a packing density of nickel hydroxide of approximately 600mAh/cc and a theoretical capacity of 3.5 Ah. Half-cells were thenprepared by placing one plate of the respective nickel positiveelectrodes thus prepared, via separators, between two plates of a knownhydrogen storage alloy negative electrode having a theoretical capacitygreater than that of the positive electrode. An aqueous solution ofpotassium hydroxide having a specific gravity of 1.30 was used as anelectrolyte solution in each half-cell.

The half-cells thus prepared were subjected to repeated charging anddischarging cycles at a temperature of 20° C. The depth of charge was120% at five-hour rate (0.7A). Discharge was continued until the cellvoltage was lowered to 0.1 V at a constant resistance corresponding totwo-hour rate (1.75 A).

FIG. 1 shows the discharge voltage plotted against the lattice constant"d" of the (002) plane of carbon powder when the cell was discharged to1.75 Ah at the constant resistance after 200 cycles. It is found clearlyfrom FIG. 1 that the cells containing carbon powder with the (002)-planelattice constant "d" such that 3.85 Å<d≦3.45 Å have high dischargevoltages.

A positive electrode "a" in accordance with the present invention wasprepared by the process above with carbon powder having the (002)-planelattice constant "d" of 3.41 Å.

A positive electrode "b" containing conventional carbonyl nickel powderinstead of the carbon powder and another positive electrode "c"containing no carbon powder were prepared as comparative examples in thesame manner as above.

A sealed battery was configured with one of the above positiveelectrodes and a hydrogen storage alloy negative electrode prepared inthe following manner. A hydrogen storage alloy represented by theformula: MmNi₃.55 Mn₀.4 Al₀.3 Co₀.75 (where Mm is a misch metalcontaining 10% by weight of lanthanum) was kneaded with water to apaste. The paste was then applied to a foamed nickel substrate used as asupport having a thickness of 1.0 mm, a porosity of 93% and a surfacedensity of 600 g/m². The substrate filled with the paste was dried,pressure-molded, and cut into a hydrogen storage alloy negativeelectrode having a size of 90×70 mm and a thickness of 0.6 mm, with apacking density of nickel hydroxide of approximately 1,280 mAh/cc and atheoretical capacity of 4.5 Ah. Each test battery included ten plates ofthe positive electrode and eleven plates of the negative electrode.

Each sealed battery had a configuration described hereinafter. As shownin FIG. 2, negative electrodes 2 and positive electrodes 8 were laid oneupon another via separators 1 of sulfonated polypropylene non-wovenfabrics. The negative electrodes 2 were placed on either end of theelectrode-layer. Leads of the negative electrodes 2 were connected to anegative terminal 4 made of nickel whereas leads of the positiveelectrodes 3 were connected to a positive terminal (not shown) made ofnickel by spot welding. The layered plates were inserted in a case 5(height: 108 mm, length: 69 mm, width: 18 mm) of acrylonitrile-styrenecopolymer resin having a thickness of 3 mm. The case 5 was then partlyfilled with 63 cc of an aqueous solution of potassium hydroxide having aspecific gravity of 1.3 used as an electrolyte solution. A sealing plate7 of acrylonitrile-styrene copolymer resin with a safety valve 6 whichis operable at two atmospheric pressure was fixed to the case 5 with anepoxy resin. The positive terminal and the negative terminal 4 wereair-tightly attached to the sealing plate 7 via O rings 8 and nuts 9.

Three sealed nickel-hydrogen storage batteries A, B and C thus preparedto have a theoretical capacity of 35 Ah respectively included thepositive electrodes "a", "b" and "c".

The batteries A, B and C were tested for cycle life in an atmosphere of20 C. The depth of charge was 120% at five-hour rate (7 A) whereas thedepth of discharge was 100% at two-hour rate (17.5 A).

FIG. 3 shows the relationship between the utilization and the number ofcharging and discharging cycles in each of the batteries A, B and C. Thebattery A had the utilization of from 93 to 95% until 1,000 cycles. Inthe battery C, on the other hand, the utilization was approximately 90%until 500 cycles and then abruptly dropped. In the battery B, theutilization was approximately 91% until 700 cycles and then abruptlydropped.

These results clearly show that the battery A in accordance with theinvention has a sufficiently long cycle life even under charge anddischarge conditions with the large discharge depth. Such a favorableeffect is not obtained under the deep discharge conditions withoutcarbon powder having the (002) plane lattice constant specified asabove. The carbon powder having the (002) lattice constant specified asabove inhibits oxidation of carbon (C+O₂ →CO₂) at the time of charging,thus maintaining high conductivity and preventing generation of γ-nickeloxyhydroxide, a non-dischargeable substance.

EXAMPLE 2

Nickel hydroxide powder, cobalt powder, cobalt hydroxide powder, carbonpowder having a variety of (002) plane-lattice constants "d" andstrontium hydroxide powder were weighed at a weight ratio of 100:3:2.5:4:1, sufficiently mixed with each other and kneaded with water toa paste. At a subsequent step, the paste was applied to a foamed nickelsubstrate used as a support having a thickness of 1.6 mm, a porosity of95% and a surface density of 600 g/m². Each substrate filled with thepaste was dried, pressure-molded, and immersed in an aqueous dispersionof a fluorocarbon resin powder. The immersed substrate was dried again,and cut into a nickel positive electrode having a size of 90×70 mm and athickness of 0.9 mm, with a packing density of nickel hydroxide ofapproximately 600 mAh/cc and a theoretical capacity of 3.5 Ah.Half-cells were then prepared by placing one plate of the respectivenickel positive electrodes thus prepared, via separators, between twoplates of a known hydrogen storage alloy negative electrode having atheoretical capacity greater than that of the positive electrode. Anaqueous solution of potassium hydroxide having a specific gravity of1.30 was used as an electrolyte solution in each half-cell.

These half-cells were tested under the conditions of 15-hour charging atten-hour rate (0.35 A) and discharging to a cut off voltage of 1 V atfive-hour rate (0.70 A) and 20° C. Charging was conducted at atmospherictemperatures of -20° C., 0° C., 20° C. and 45° C. (hereinafter referredto as charge temperature).

FIG. 4 shows the relationship between the utilization and the (002)plane lattice constant of carbon powder included in each positiveelectrode at the various charge temperatures. It is found from FIG. 4that the positive electrodes containing carbon powder with the(002)-plane lattice constant "d" of greater than 3.35 Å and not greaterthan 3.45 Å have high utilization.

A positive electrode "d" in accordance with the invention was preparedby the process above with carbon powder having the (002)-plane latticeconstant "d" of 3.41 Å.

A positive electrode "e" containing no strontium hydroxide powder,another positive electrode "f" containing no carbon powder, and stillanother positive electrode "g" containing neither strontium hydroxidepowder nor carbon powder were prepared as comparative examples in thesame manner as above.

Half-cells were prepared with these positive electrodes "d", "e", "f"and "g" in the same manner as above, and then tested under the sameconditions as above.

FIG. 5 shows the relationship between the charge temperature and theutilization in the variety of positive electrodes. As clearly shown inFIG. 5, the positive electrode "d" in accordance with the inventionexerted excellent properties in a wide temperature range. Both strontiumhydroxide powder and carbon powder were found essential for thesufficient utilization. Strontium hydroxide powder included in thepositive electrode improved the characteristics at high temperatures buthad poor conductivity in the range of low through ordinary temperatures.Carbon powder included in the positive electrode, on the other hand,improved the characteristics in the range of low through ordinarytemperatures but had poor conductivity at high temperatures.

The same test was conducted for a variety of positive electrodescontaining, in place of strontium hydroxide powder used in this example,Ca(OH)₂, CaO, CaF₂, CaS, CaSO₄, CaSi₂ O₅, CaC₂ O₄, CaWO₄, SrCO₃, BaO,Cu₂ O, Ag₂ O, Y₂ (CO₃)₃ or Y₂ O₃. In any positive electrode thusprepared, the utilization was 70% or more in charging at 45° C. Incharging at -20° C., 0° C. and 20° C., the utilization was not less than82%, not less than 88% and not less than 93%, respectively.

EXAMPLE 3

Nickel hydroxide powder, cobalt powder, cobalt hydroxide powder, nickelpowder having a variety of specific surface areas and average particlediameters, and yttrium oxide powder were weighed at a weight ratio of100:3:2.5:4:1, sufficiently mixed with each other and kneaded with waterto a paste. At a subsequent step, the paste was applied to a foamednickel substrate used as a support having a thickness of 1.6 mm, aporosity of 95%, and a surface density of 600 g/m². Each substratefilled with the paste was dried, pressure-molded, and immersed in anaqueous dispersion of fluorocarbon resin powder. The immersed substratewas dried again, and cut into a nickel positive electrode having a sizeof 90×70 mm and a thickness of 0.9 mm, with a packing density of nickelhydroxide of approximately 600 mAh/cc and a theoretical capacity of 3.5Ah. Half-cells were then prepared by placing one plate of the respectivenickel positive electrodes thus prepared, via separators, between twoplates of a known hydrogen storage alloy negative electrode having atheoretical capacity greater than that of the positive electrode. Anaqueous solution of potassium hydroxide having a specific gravity of1.30 was used as an electrolyte solution in each half-cell.

These half-cells were tested under the conditions of 15-hour charging atten-hour rate (0.35 A) and discharging to a cut off voltage of 1 V atfive-hour rate (0.70 A) and 20° C. Charging was conducted attemperatures of -20° C., 0° C., 20° C. and 45° C.

FIG. 6 shows the relationship between the utilization and the specificsurface area (0.05 to 10 m² /g) of nickel powder included in eachpositive electrode at various charge temperatures. FIG. 7 shows therelationship between the utilization and the average particle diameter(0.05 to 18 micrometer) of nickel powder included in each positiveelectrode at various charge temperatures. It is found from FIG. 6 andFIG. 7 that the positive electrodes containing nickel powder having thespecific surface area of from 0.1 to 3 m² /g and the average particlediameter of from 0.1 to 15 micrometer have high utilization.

A positive electrode "h" according to the invention was prepared by theprocess above with nickel powder having a specific surface area of 2 m²/g and an average particle diameter of 5 micrometer. A positiveelectrode "i" containing no yttrium oxide powder, another positiveelectrode "J" containing no nickel powder, and still another positiveelectrode "k" containing neither yttrium oxide powder nor nickel powderwere prepared as comparative examples in the same manner as above.

Half-cells were prepared with these positive electrodes "h", "i", "J"and "k" in the same manner as above, and then tested under the sameconditions as above.

FIG. 8 shows the relationship between the charge temperature and theutilization in the variety of positive electrodes. As clearly shown inFIG. 8, the positive electrode "h" according to the invention exertedexcellent properties in a wide temperature range. Both yttrium oxidepowder and nickel powder are essential for the sufficient utilization.

Another powdery compound, such as Ca(OH)₂, may be used in place of theyttrium oxide powder to have the equivalent characteristics.

Sealed nickel-hydrogen storage batteries having excellentcharacteristics as that of Example 1 can be prepared by respectivelycombining the positive electrodes of Example 2 and Example 3 with thehydrogen storage alloy negative electrode described above.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosures is not to be interpreted as limiting. Various alterationsand modifications will no doubt become apparent to those skilled in theart to which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

What is claimed is:
 1. A nickel positive electrode for an alkalinestorage battery comprising:an active material mixture mainly composed ofnickel hydroxide and a conductive support for said active materialmixture, said active material mixture containing at least one memberselected from the group consisting of cobalt, cobalt hydroxide andcobalt oxide, and carbon powder having a lattice constant "d" of a (002)plane such that 3.35 Å<d≦3.45 Å.
 2. A nickel positive electrode inaccordance with claim 1, wherein said active material mixture furthercontains a powdery compound of at least one element selected from thegroup consisting of Ca, Sr, Ba, Cu, Ag and Y.
 3. A nickel positiveelectrode in accordance with claim 2, wherein said powdery compound isat least one member selected from the group consisting of Ca(OH)₂, CaO,CaF₂, CaS, CaSO₄, CaSi₂ O₅, CaC₂ O₄, CaWO₄, SrCO₃, Sr(OH)₂, BaO, Cu₂ O,Ag₂ O, Y₂ (CO₃)₃ and Y₂ O₃.
 4. A nickel positive electrode in accordancewith claim 1, wherein the content of said carbon powder included in saidactive material mixture is from 0.1 to 8 parts by weight per 100 partsby weight of said nickel hydroxide.
 5. A nickel positive electrode inaccordance with claim 1, wherein said carbon powder is flake graphitepowder.
 6. A nickel positive electrode in accordance with claim 3,wherein the content of said powdery compound included in said activematerial mixture is from 0.1 to 5 parts by weight per 100 parts byweight of said nickel hydroxide.
 7. A sealed nickel-hydrogen storagebattery comprising a nickel positive electrode, a negative electrodecomprising a hydrogen storage alloy which electrochemically absorbs anddesorbs hydrogen, a separator, an electrolyte solution consisting of analkaline aqueous solution and a sealed container having positive andnegative terminals and a resettable safety valve,said nickel positiveelectrode comprising an active material mixture mainly composed ofnickel hydroxide and a conductive support, said active material mixturecontaining at least one member selected from the group consisting ofcobalt, cobalt hydroxide and cobalt oxide, and carbon powder having alattice constant "d" of a (002) plane such that 3.35 Å<d≦3.45 Å.
 8. Asealed nickel-hydrogen storage battery in accordance with claim 7,wherein said active material mixture of said nickel positive electrodefurther contains a powdery compound of at least one element selectedfrom the group consisting of Ca, Sr, Ba, Cu, Ag and Y.
 9. A sealednickel-hydrogen storage battery in accordance with claim 8, wherein saidpowdery compound is at least one member selected from the groupconsisting of Ca(OH)₂, CaO, CaF₂, CaS, CaSO₄, CaSi₂ O₅, CaC₂ O₄, CaWO₄,SrCO₃, Sr(OH)₂, BaO, Cu₂ O, Ag₂ O, Y₂ (CO₃)₃ and Y₂ O₃.
 10. A nickelpositive electrode in accordance with claim 2, wherein the content ofsaid carbon powder included in said active material mixture is from 0.1to 8 parts by weight per 100 parts by weight of said nickel hydroxide.11. A nickel positive electrode in accordance with claim 2, wherein saidcarbon powder is flake graphite powder.